In the normal or power providing operation of reciprocating internal combustion engines, exhaust occurs near the end of the power stroke where the combustion gas pressure is relatively low after expansion of the combustion gases. For compression braking, however, fuel is not provided and exhaust of compressed air occurs near the end of the compression stroke; and, in order to dissipate as much of the energy of compression as possible, the cylinder pressure at exhaust valve opening is much higher than in normal operation. As a result and for a poppet exhaust valve utilizing a spring to close the valve, the force for valve opening during compression braking operation is, typically, 3 to 8 times that for normal operation.
For engines having poppet valves providing these operations, hydraulically motivated and electronically controlled actuation of the valves is desirable to provide flexibility in timing for optimized response, power, noise control, efficiency, and pollution control. Many arrangements dispensing with cams and providing such actuation are possible and a number are well-known. Since these arrangements are not a part of the present invention, it will only be mentioned that such arrangements to which the described embodiment of the present invention is applicable utilize a source of low hydraulic pressure and a source of high hydraulic pressure selectively connected to an actuator for closing and opening each poppet valve at varying and optimal times in relation to the position of the engine crankshaft. The hydraulic source selection is, typically, performed by electrically motivated valves and is electronically determined by a control system sensing variables such as crankshaft rotational speed and position, air inlet pressure, desired power in normal operation, and desired amount of compression braking. The control system provides, in relation to crankshaft position, the different exhaust poppet valve opening timing for normal operation and compression braking.
However, typical hydraulically motivated poppet valve actuators lack the force capability to open the exhaust valve during compression braking due to the above mentioned large required force. While actuators could be constructed to provide the necessary force, this would result in wasted energy for valve actuation in normal operation. It would, of course, be possible to construct such actuators providing the appropriate required different forces for normal operation and braking as selected by a control system of the nature described above. However, both the actuator and control system would still not be directly receptive to an actual need for greater force and would thus be somewhat inefficient. In any event, such an actuator and control system would necessarily be more complex, and the actuator could not directly replace existing camless hydraulic actuators lacking the force capability to open an exhaust valve during a compression braking cycle.